My goal was to have every car in every color. I was bitterly disappointed when I hit 1000 and the game wouldn't let me buy any more. It's so arbitrary - would someone please hack this...
I have revised my goal to having every prize car in every possible color, plus every 6.2 mile car.
As of now (99.6% complete, 42 Golds):
I have at least one of every car (excepting the last few prize cars, of course)
I have 3 sets of black GTP cars.
I have all 15 colors of the Mercury Cougar XR-7.
I have 10 of the 11 Formula GT car colors (13 cars total).
I have about 300 6.2 mile cars.
I have 105,196,180 credits (for a rainy day).
In real life, I want a garage with attached house:
'02 Honda VFR
'96 Camaro Z28
'95 GMC Safari AWD
'93 30th Anniversary Corvette 6sp
'63 Corvette Coupe
(gave my '86 944 turbo to the kids)
As I proofread this, I see that I am clearly INSANE!